Vinyl halide polymers, particularly polyvinyl chloride resins, are used in a variety of applications. These thermoplastic polymers can be fabricated into useful articles by extrusion, injection molding, compression molding and other thermoforming methods. Generally, these methods involve mixing the resins with certain additives to form compositions, heating the compositions to a temperature to fuse the resin particles, forming the compositions into the desired shape, and then cooling the composition to a solid. Due to the presence of unsaturation and labile allylic chlorines in the polymer backbone, the resins are sensitive to heat exposure and the resins are also sensitive to aging. The resins do not exhibit a distinct melting point and must be combined with fusing agents to facilitate their fusing during forming. At the temperatures that would be expected to melt the resins, the resins degrade and turn black. The polyvinyl chloride (PVC) resins degrade because the allylic chlorines are activated when exposed to heat and are released from the polymeric backbone. A free radical remains that then can react with another part of the backbone causing crosslinking, which renders such polymers unprocessable. Also, chlorine free radicals react together to form chlorine or with released hydrogen to form hydrogen chloride. Although PVC resins are initially white, thermoformed PVC articles that are damaged by heat during processing can range in color from yellow to black.
To stabilize vinyl halide polymers during thermoforming methods, heat or thermal stabilizers and costabilizers are added to the resins. By using these stabilizers, the resin can be fused with a reduced degree of degradation and discoloration. Examples of known stabilizers include organometallic stabilizers, mixed metallic stabilizers, and inorganic stabilizers. Metallic stabilizers include heavy metal, alkali metal, and alkaline earth metal salts of fatty acids. The heavy metals include lead, cadmium, tin, and zinc. Popular organometallic stabilizers are dibutyltin maleate and dibutyltin di-2-ethylhexanoate. Examples of mixed metallic stabilizers include fatty acid salts of zinc and calcium, barium and cadmium, lead and barium, or two or more of other metals. Examples of inorganic stabilizers include lead carbonate, lead sulfate, and mixtures of tribasic lead sulfate and dibasic lead stearate. Examples of organic, non-metallic stabilizers or costabilizers include epoxidized soybean oil, tri(nonylphenyl) phosphite, .beta.-aminocrotonates, .beta.-diketones, and phenylindole.
One has only to observe the attempt to process a vinyl halide resin, such as resin, in the absence of any stabilizer to become a believer in the necessity for stabilization. Heating unstabilized PVC resin above its fusion point initially gives rise to yellowing, followed quickly by gross discoloration, evolution of hydrochloric acid, crosslinking, and ultimate charring to an infusible, unprocessable, corrosive black mass.
An article in Die Angewandte Makromolekulare Chemie. 168 (1989) 23-35 (Nr. 2718) discloses thermal stabilization of rigid PVC using maleimides. The article is by M. W. Sabaa, M. G. Mikhael, N. A. Mohamed, and A. A. Yassin of Cairo University and discloses fourteen aryl maleimides and one alkyl maleimide for thermal stabilization of rigid PVC. It is believed that this article leads away from consideration by those skilled in the art of N-alkyl maleimides as suitable stabilizers for rigid PVC because of poor performance of N-alkyl maleimides compared to the N-aryl maleimides that were tested. It is believed that the poor results obtained were due to the testing procedure employed in evaluating thermal stabilization, which testing procedure did not properly reflect the efficacy of the N-alkyl maleimide as a thermal stabilizer for rigid PVC.